The use of single layer anti-reflective coatings to decrease the reflectance of the interface between two propagation media is well known. A coating which satisiies a well-known equation involving the indices of refraction of the coating, the substrate and the overlying medium, the wavelength of the energy and the thickness of the coating is typically used to minimize the reflectance of the interface.
In the field of optically sensitive semiconductor devices such anti-reflective coatings have been widely applied to photovoltaic cells and other discrete devices. A common coating used for this purpose is silicon nitride ( Si.sub.3 N.sub.4). This material is attractive because it is readily available and because it has an index of refraction lower than that of silicon.
Currently, integrated circuits in which an optically sensitive device is combined with a number of other circuit elements are becoming increasingly important. In order to apply an anti-reflective coating to the optically sensitive device in such circuits, it is necessary to add extra mask and process steps near the end of the manufacturing process flow in order to remove thick overlying passivation layers and then to apply the anti-reflective coating. In most cases, the added cost of these extra steps is not recoverable due to the relatively low margins available on these integrated circuits. Therefore, no anti-reflective coating is commonly applied to such devices.